Modern medical techniques have made it possible to conduct non-invasive testing of the human body to detect a variety of internal growths and abnormalities. Specifically, magnetic medical imaging equipment that employs huge electromagnets can scan the entire human body or any desired part thereof to assist the physician in diagnosing patient ailments. Usually, these huge electromagnets are fabricated from coils of superconductor metal foil such as a tin coated niobium alloy foil which contains a niobium tin superconductor alloy. Although the process for fabricating these coils has been used in industry for quite some time, there have been unsolved problems in the prior production processes.
Long-length niobium alloy foil is required to produce the superconductor windings for the electromagnet used in medical imaging equipment. Typically, "long length" is at least 3,000 feet long although "long length" can be as long as 9,000 feet. Foil thickness is typically in a range of 0.005 inches and 0.0008 inches. The prior art method for producing a niobium metal alloy foil begins by forming a workpiece slab of preferably niobium 1% by weight zirconium alloy metal having a thickness of approximately 3 inches into a sheet of niobium zirconium alloy metal having a thickness of approximately 0.040 inches. Typically, this is achieved on a standard 4-high cold mill. Thereafter, a conventional foil mill such as a sendzimiror Z-mill is used to compress the 0.040 inch niobium zirconium alloy metal sheet to a final foil gauge thickness such as 0.010 inches or some other selected gauge thickness. The Z-mill is typically one of the kind manufactured by Waterbury Farrel Founding and Machine Company of Waterbury, Conn.
The compressed sheet is simultaneously collected and rolled into a coil in one of two coil assemblies disposed on opposite sides of the rollers depending upon the direction that the sheet passes through the rollers. 0f course, as one coil assembly receives the compressed sheet from the rollers the opposite coil assembly discharges the sheet by unwrapping the coil collected therein. Typically, each these coil assemblies applied a high tension force to the sheet to achieve the required foil gauge thickness. The receiving coil assembly applied a high receiving tension force to the sheet as it is being received from the rollers thereby inducing a pulling force to the sheet into the receiving coil. The discharging coil assembly applied a discharge tension force to the sheet which is equal in magnitude to the receiving tension force as the sheet was discharged from the discharging coil assembly thereby inducing a force which resisted being pulled by the rollers. These tension forces can cause the sheet to crease and wrinkle which is undesirable when forming a refractory metal foil. Also, these tension forces caused an inconsistent cross-sectional sheet profile in the foil.
Typically, the workpiece slab is forged into a thickness of approximately 3 inches. Forging usually causes oxide compounds to form along the lateral edges of the slab. As the workpiece slab is rolled i.e., compressed and reduced to a sheet, these oxide compounds can form inclusions which can cause cracking along the lateral edges of the sheet during further reduction processing. This cracking can subsequently cause catastrophic breakage of the foil.
One manufacturer of long-length refractory metal foil attempted to produce a long-length foil by welding two foils together along their ends at a stage where the thicknesses of the foils were 0.040 inches. This weld was approximately one inch in length that connected the two foils. After reducing the thickness of the "long-length" foil to 0.001 inches, the weld became 40 inches in length. Furthermore, this 40-inch weld often contained numerous pinholes throughout its surface area and could later break. Breakage could be particularly catastrophic after processing the foil into a superconductor surface.
Once the sheet of niobium zirconium alloy foil was reduced in thickness to approximately 0.040 inches and rolled into a coil, the coil was annealed in a furnace for 2 hours at 2,200.degree. Fahrenheit. In production processes, this annealing process can produce a coil with un-annealed surface areas. Coils that are un-annealed develop areas of excessive dislocation stress throughout the metal matrix during subsequent rolling. Generally, these un-annealed areas are located in the middle of the coil, the location that is the most difficult to anneal. Upon subsequent rolling to a thinner thickness, breakage of the foil at these locations can occur.
After annealing the coil must be mounted onto the Z-mill once again so that the foil could be rolled to a smaller thickness. As the coil becomes unwrapped after annealing, sometimes the facially contacting surfaces of the foil stick together, thus, destroying the foil. It is believed that non-homogeneous annealing causes the facially contacting surfaces of the foil to stick together.
All of these problems either individually or in combination with each other led to an inability to consistently produce a high quality, dimensionally consistent long-length foil. Therefore, there is a need in the industry to produce a refractory metal foil from a workpiece slab of refractory metal which consistently yields a desirable sheet thickness and shape, as well as a quality, long-length refractory metal foil. It would be advantageous if the workpiece slab of refractory metal is subjected to homogeneous annealing to prevent un-annealed areas in the middle of the coil. It would also be advantageous if the oxide compounds are first removed from the lateral edges of the workpiece to prevent edge cracking and subsequent catastrophic breakage of the foil. There is need in the industry to produce a refractory metal foil that is devoid of excessive dislocation stress. It would be advantageous if a long-length, i.e. greater than 3,000 feet, refractory metal foil could be produced without having to weld two sheets together. There is also a need in the industry to produce a refractory metal foil so that it could be rolled into a coil for annealing purposes and thereafter unwrapped for further reduction in thickness without the facially contacting surfaces sticking together. There is a need in the industry for method to produce a refractory metal foil that employs a lower tension methodology when compressing the refractory metal in a Z-mill into a reduced thickness. It would also be advantageous if a sheet receiving tension force is generated that is slightly less than a sheet discharging tension force. The present invention satisfies these needs and provides these advantages.